Introduction to Turbines

7/28/2019 Introduction to Turbines

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Introduction to Turbines & Steam Turbines

FUNDAMENTAL PRINCIPLES AND OPERATION

A. THE WINDMILL

Turbines have been in use for many years. Early turbines consisted of 'Sails' or blades mounted at

an angle on a central hub (as in the child's pin-wheel in Figure: 1).

The hub was then connected to a shaft and, as the wind blew, due to the angl e of the sails, the sails

rotated causing the hub to rotate and so turned the shaft. The shaft was then coupled to a ' Mill -

Wheel ' used for gr inding corn to make flour and other uses.


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(Picture 1).

Windmills were also used to drive water pumps which drew water from wells for farming irrigation

and domestic use. In order to keep the sails facing into the wind, the windmill was fitted with a

'Rudder' which swung the sails around as the wind direction changed. (Picture 2).


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When the generation of electricity was introduced, windmills were adapted to drive generators. One

problem with a windmill is that it depends on there being sufficient wind to drive it. Today, modern

'Wind-farms' have been developed with highly technical machines for producing large quantities of

electricity, even when the wind velocity is low. (PICTURE: 3)

Picture: 3 - Modern Wind-farm


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B. THE WATERWHEEL ( Figure. 2 )

The waterwheel has also been in use for many years. In this type of turbine, a wheel fitted with '

Paddles ' is placed in a stream of flowing water. As the water pushed against the paddles, the wheel

rotated which, in turn, through a shaft, rotated another machine - grinding mill, pump or generator.

Again, the efficiency of the waterwheel depended on the velocity and volume of water striking the

paddles.

[

(* POTENTIAL ENERGY * - Stored Energy) - Energy waiting to be used.

(* KINETIC ENERGY * - Energy due to motion). This high energy water is piped to large ' Water

Turbines ' which drive the power generating plants. (See Figure. 3).


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It can be seen that a 'TURBINE' is a machine which is used as a driver for other machines -

Generators, pumps, compressors...Etc. A turbine operation depends on the Kinetic energy contained

in flowing fluids which is then converted into Mechanical energy. This Mechanical energy is thenconverted into Electrical, Heat (Thermal) or Pressure energy as required. A turbine therefore, is used

in the same way as a Diesel engine, Petrol engine or Electric motor, to drive other machines.

Modern turbines can produce thousands of Horse-power of energy.

STEAM TURBINES

INTRODUCTION

As already stated, a turbine is driven by the flow of a high energy fluid - liquid, gas (or air). Thekinetic energy of the fluid is converted into mechanical energy. In steam turbines, the thermal and

pressure energy contained in superheated, high pressure steam is used to drive the shaft of the

turbine. Steam turbines are generally used where there is a plentiful supply of water. The water must

first be treated to remove impurities which will cause problems in the turbine. - Chlorides, other salts,

Oxygen and solid particles. This is done to prevent corrosion, erosion and scale deposits in the

system. When the water has been purified, it is then passed into a Steam Generation Plant where it

is heated to produce steam. Steam at normal atmospheric conditions is Saturated (Wet) steam - i.e.


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212 F (100 C) and is of no use for driving turbines. In the type of boiler used for steam generation,

the system is maintained under high pressure - In this discussion we will use a steam system

operated at 600 Psi. At this pressure, the water boils at 486 F. However, at this pressure and

temperature the steam is still saturated (wet steam). The use of this steam in a turbine will cause

erosion of the turbine internals due to droplets of water contained in the steam. The boilers are

therefore constructed with a 'Super-heater' section which takes the 600 Psi wet steam and adds

more heat energy to it, to a temperature of 775 F or higher depending on requirements. At this

temperature, the steam cannot contain any water. When steam is super-heated, it contains much

more heat energy than wet steam and can be piped long distances with little loss of energy or

condensation taking place.

To re-cap, the steam used for driving steam turbines is produced from purified water to prevent

corrosion and is produced at high pressure and super -heated to high temperature in order to prevent

water erosion of the turbine parts. There are many types of steam turbine in use today which can

produce many thousands of horse-power of energy for industrial uses.

PRINCIPLES & OPERATION

In the pin-wheel, the windmill and the water wheel, the action of the flowing fluid causes the wheel to

rotate. This part of the machine is called the 'ROTOR'. In any turbine, the rotor is mounted on a shaft

and consists of the 'Sails' or 'Paddles' which we will now refer to as 'Blades'. The blades are fitted

into a wheel at an angle and are called 'Rotor Blades'. The wheel is then mounted on to the shaft.

This arrangement of a single wheel is called 'one stage' or a 'Single Stage Rotor' and does not

produce high power. (See Figure. 4)


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Figure. 4 - Single Stage Turbine

For large processing and generation plants, very powerful turbines are needed for driving machines

like compressors, large pumps or generators. In this case, 'Multi-Stage Turbines' are used. As stated

earlier, the energy needed to drive these turbines, comes from high pressure, superheated steam. In

order to get the steam to pass to the rotor blades, we need a means of directing the steam on to the

blades. The piece of equipment used for this is called a NOZZLE. As the steam leaves a nozzle, its

pressure decreases and its 'VELOCITY' increases. This high velocity steam jet is directed at the

rotor blades and, as in the pin-wheel, the rotor and shaft begins to rotate. As more and more steam

is released on to the blades, the speed of rotation increases. (As with a windmill, stronger wind,

faster rotation).

MULTI-STAGE STEAM TURBINES

A Multi-stage turbine is one which has two or more wheels. The steam is directed on to the blades of

the first stage wheel and, as it strikes the angled blades, they move away in an opposite direction to

the flow of steam, causing rotation of the wheel and shaft. As the steam gives up energy to move the

rotor blades, its pressure is decreased, its volume increases and it leaves the blades in an opposite

direction to that taken by the wheel.(See Figure. 5)

Some method of re-directing the steam on to a second wheel is now needed. To achieve this, a row

of fixed, unmoving, angled blades is fitted into a DIAPHRAGM which is mounted in the 'CASING' of

the turbine. These stationary blades are called ' STATOR BLADES '.

Figure: 5 - Steam Flow Through the Rotor & Stator

The stator blades act like further nozzles and re-direct the steam on to the rotor blades of the second

stage wheel. Because the steam pressure has dropped and its volume is greater, to get the same


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amount of energy out of it, the blades of the second stage are larger (greater area) than those of the

first stage. This arrangement of alternating rotor and stator blades and increasing blade size, is

continued through the turbine in order to obtain the required amount of power from the steam for

operational needs. When the steam leaves a turbine, it may still contain a lot of energy - pressure

and heat. This steam may be directed for use in another process system. (See Figure. 6)


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(Figure. 7) - Shows a typical lube oil system for a steam turbine.

The lube oil system comprises a reservoir or oil tank in which three pumps are immersed. The main

oil pump is driven by a shaft from the Steam Turbine accessory gear. The Auxiliary pump is driven

by an A/C motor and is used for start-up, shut-down and other operating conditions necessitating its

use. The 3rd pump is operated by a D/C motor (battery supplied) for use on main power failure -

shut down of the complete system which will require lube oil while the units shut down. From the

pumps the oil at the required pressure (controlled by PCV 1 that spills excess back to the reservoir),

passes through 1 of 2 water cooled exchangers (1 operating & 1 standby) and temperaturecontrolled by a TCV. After cooling the main oil flow passes through 1 of 2 filters (1 operating & 1

standby). The filters are fitted with a Differential Pressure (DP) gauge and alarm which, should the

filter begin to get too dirty, at a pre-set DP will warn that the filters need changing over and the dirty

elements changed out. From the filters the oil passes via a control valve (PCV 2) which maintains

the desired lube oil pressure to the bearings of the turbine and possibly also to its driven machine -

Compressor, Generator, pump .. etc. After lubricating and cooling the bearings, the oil returns to the

reservoir. Any oil losses are made up via the oil make up line to the reservoir. In the oil systems, a


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number of alarms and shut-down devices are installed to ensure the safe operating conditions for

the machine. Hydraulic Oil is provided from the lube oil system from between the coolers and the

filters. This oil may be boosted in pressure, filtered and pressure controlled by PCV/A and is used for

the control and shut-down systems of high power steam turbines.

STEAM CONTROL TO A TURBINE

In order to control a turbine speed, a method of controlling the steam supply is needed. To do this,

the turbine steam inlet first enters a 'STEAM CHEST'. (Figure. 8) The steam chest contains a se ries

of steam valves which can be opened gradually as required. As each valve opens the flow of steam

to the nozzle(s) is increased thus increasing the turbine speed.

Figure 8

CONTROL SYSTEM DESCRIPTION


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(See Figure 9).

The hydraulic/control oil enters the unit and is piped to the following parts of the system. (The oil

pressure depends upon the maker's specifications).

1. Power Piston

2. Trip and Throttle Valve

3. Overspeed Trip Mechanisms & Slide Valves 'A' and 'B'

4. Trip Solenoid Valve

1. The Power Piston - The oil passes to the power piston via Port 'Y' of Slide Valve 'A'. The

Governor, on the signals from the control system will, through a Servo-Mechanism', adjust the

hydraulic oil to the power piston. This in turn, controls the steam flow via the steam chest valves to

the nozzles, thus controlling the turbine speed.

2. The Trip and Throttle Valve - This valve passes the H.P. steam to the steam chest. On startup ofthe turbine, the steam chest valves are fully open. The T/T valve is opened slowly by a hand-wheel

until the machine comes to Minimum Governor Control setting - - the power piston falls and the

steam supply to the nozzles comes under Governor control. When the turbine is under the control of

the governor, the T/T valve hand-wheel is swung to the fully open position. (This will not increase

steam flow to the turbine due to the governor control of the steam chest valves). The high pressure

hydraulic oil passes to the T/T valve cylinder via a restriction orifice and the ' Y ' Port of Slide Valve '

B '. This oil pressure acting on the piston keeps the main steam valve fully open during normal

operation. In order to periodically check the operation of the T/T valve, a 25% stroke solenoid

operated valve is fitted. On operation of the stroke check button, the valve is energised and bleeds

off an amount of oil from the T/T valve cylinder. The T/T valve closes down by 25% without affectingthe steam flow to the turbine. When the check button is released, the T/T valve goes to the fully

open position again.

3. The Overspeed Trip Mechanism & Slide Valves ' A ' and ' B '

The O/S trip, as its name implies, is a Mechanical shutdown device in the event of turbine excessive

speed. (Overspeed trips are discussed later). The slide valves are kept in the ' RUN ' position by

applying oil pressure to the valve piston against a return spring. The oil feed to these mechanisms

also passes through a restriction orifice. From this feed line, oil is also piped to the 'Trip Solenoid

valve.

4. The Trip Solenoid Valve - This is an Electrical shut-down device which receives a signal from the

electrical trip circuit which includes - High vibration, Low lube oil pressure, High bearing temperature,

Low hydraulic oil pressure ... etc. The electrical signal energises the solenoid which opens the valve

and dumps the hydraulic oil back to the reservoir. The oil pressure is dumped to zero Psi due to the

oil flow rate through the restriction orifices being LESS than the flow of oil returning to the reservoir.

When the oil dumps, Slide valves ' A ' & ' B ' are pushed across by their springs. This CLOSES the

oil supply to the T/T valve and to the Power piston via the ' Y ' Ports and OPENS the ' X ' Ports to


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dump the oil. The T/T valve closes and the steam flow is stopped. At the same time th e Power piston

rises to fully open the steam chest valves. (No steam can flow as the T/T valve has closed). Before

resetting the trip condition - electrical or mechanical, the T/T valve hand -wheel must be spun to the

closed position and made ready for start-up and the governor control system set to minimum

governor. When the trip system is re-set, the hydraulic oil pressure is restored and the two slide

valves move across to the ' GO ' position again. The machine can now be re -started.

TURBINE CONTROL SYSTEM

Figure. 9


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TYPES OF STEAM TURBINE

(Figure. 10)

For any steam turbine to operate, a pressure difference must exist between the steam supply and

the exhaust. Where the exhaust steam is above atmospheric pressure, the turbine is classed as a

Back Pressure Turbine' or 'Non-Condensing Turbine'.

I. BACK-PRESSURE STEAM TURBINES

As an example, taking a 600 Psi steam supply to a turbine, the turbine speed is controlled by the

steam input. If we have an exhaust pressure of say 125 Psi (a D.P. of 475 Psi), this exhaust steam

will still contain a lot of heat and pressure energy and may be used to drive other smaller turbines

and for heating purposes in re-boilers, heaters, vaporisers...etc. In this type of turbine, the exhaust

must be maintained at a constant pressure by a PCV control system downstream of the turbine

exhaust to prevent changes in the exhaust pressure that would affect the turbine speed by changing

the pressure drop across it. The governor would be fighting against these pressure fluctuations and

speed control would be erratic.

II. CONDENSING STEAM TURBINES

In a condensing steam turbine, the maximum amount of energy is extracted from the steam. This is

achieved by passing the exhaust steam into a condenser (called a Surface Condenser). The steam

is condensed by surface contact with bundles of tubes through which cooling water is passing. As

the steam condenses, its volume, on changing to water, decreases by about 1800 times. This great

decrease in volume causes a vacuum to form in the condenser. Due to this, the pressure drop

across the turbine and therefore the turbine power is maximised. The steam condensate (water) is

level controlled in the condenser and pumped back to the steam generation plant. However,

although the water for the steam generation is purified and treated, the steam will still cont ain some

Non-condensibles. These will build up in the surface condenser and gradually destroy the vacuum,

thereby decreasing the P.D. across the turbine and thus decreasing its efficiency and power. In

order to maintain the vacuum, the non-condensibles must be removed from the surface condenser.This is carried out by a system of STEAM EJECTORS' and Ejector Condensers' which pull the

gases from the surface condenser and eject them to the atmosphere.


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TYPES OF STEAM TURBINES

Figure. 10

REMOVAL OF NON-CONDENSIBLES

As stated, the gases are removed from the surface condenser by a system of 1st and 2nd stage

ejectors and condensers. An ejector consists of a 'Venturi Tube' through which a jet of high velocity

steam is passed. This high velocity steam creates suction (vacuum) in the Venturi tube. Vacuum is

increased by condensing the steam as it leaves the ejector. The non-condensibles are piped into the

low pressure area of the 1st stage ejector and are carried with the steam into the 1st stage ejector

condenser. The water produced from the steam is piped back to the surface condenser. Again,

these gases, as they build up in the 1st stage ejector condenser, will tend to destroy the vacuum. To


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prevent this, the gases are pulled from the 1st stage system into the 2nd stage by another ejector.

Again the steam is condensed and piped back to the surface condenser. In the 2nd stage

condenser, the gases are allowed to build up pressure until, at just above atmospheric pressure, a

check (non-return) valve will open and pass them to atmosphere. As they escape, the pressure drop

causes the check valve to close again. - This is a continuous process. A water level is maintained in

the ejector condensers by a ' Loop ' seal tube to prevent the gases also returning to the surface

condenser.

(Figure. 11)

SURFACE CONDENSER

Figure. 11

TURBINE SEAL-STEAM SYSTEM (Figure. 12)


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When a condensing steam turbine is first started up and in a low-load condition, steam from the inlet

(H.P.) end will leak from the outboard gland - even though carbon-ring seals and labyrinth seals are

installed to minimise the leakage. Superheated steam is invisible and, due to its high temperature, is

very dangerous. Leakage of steam is also a waste and is not desirable. Conversely, under the same

low-load conditions, the L.P end of the turbine will be under the vacuum of the surface condenser.

The vacuum will tend to pull in cold atmospheric air through the seals along the shaft. Cold air will

have a detrimental effect on the hot metal of the shaft which can lead to damage. In order to

minimise these problems, a manually controlled supply of low pressure SEAL steam (about 2 Psi), is

piped to a common line feeding the glands of the machine. This pressure will prevent the ingress of

air at the L.P. end and ensure a positive pressure at the H.P. end during start-up. (The ejector units

are started and vacuum pulled before starting the turbine). When the turbine load is increased, the

leakage of steam into the Seal-steam header will cause greater pressure than the Seal steam supply

and will begin to flow to the L.P. end seal. At this point, the Seal steam suppl y can be shut down and

the Seal-steam taken from the H.P. end and manually maintained at 2 Psi by venting excess into the

surface condenser. Even when the seal pressures are maintained at 2 Psi, some leakage occurs

from both seals. This is taken care of by the LEAK steam recovery system.

TURBINE LEAK-STEAM SYSTEM (Figure. 13)

Leakage of steam from turbine glands to atmosphere (in both types of turbine), as already

mentioned, is dangerous and a waste of steam. The turbine inboard and outboard glands are fit ted

with carbon ring seals and labyrinth seals to help prevent leakage. These seals do not completely

stop the escape of steam. In order to completely prevent it, a turbine will also be fitted with a LEAK-

steam system which pipes the leakage into a water cooled condenser. The condensate from the

condenser is drained away to a 'Hot-well' from where it is returned to the steam generation plant.

The leak steam condenser is fitted with an ejector system to remove non-condensibles from the

steam and discharge them to atmosphere with the ejector exhaust steam. This, in the case of thecondensing turbine, is to prevent non-condensibles from re-entering the surface condenser.

SEAL STEAM SYSTEM


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Figure. 12


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TURBINE LEAK STEAM SYSTEM

Figure. 13

Documents

Introduction to Turbines